Charge coupled devices (CCD) have traditionally been the imagers of choice for high quality imaging devices due to their high sensitivity and fill-factor. However, the standard CCD formation process often is incompatible with CMOS processing. Accordingly, it is impractical to integrate on-chip signal processing electronics in a CCD imager and the signal processing electronics must be provided off-chip.
CCDs are essentially electronic analog shift registers. CCDs include rows of photodetectors that act essentially as capacitors. Each pixel converts light energy into a charge, and then transfers that charge sequentially across a row of pixels into a single charge amplifier. This operation requires nearly perfect charge transfer efficiency, resulting in slow readout rates and relatively large power requirements.
Active pixel sensor (APS) devices are described in U.S. Pat. No. 5,417,515. These include an array of pixel cells that convert light energy into electrical signals. Each pixel includes a photodetector and one or more active regions. APS imaging devices are compatible with complementary metal oxide semiconductor (CMOS) technology.
Conventional CMOS imaging devices can be prone to producing artifacts in the resulting image. Artifacts are areas in the image that correspond to areas of the sensor that have been exposed to extremely high light levels. Artifacts appear black in the image when in fact they should be the brightest objects in the image.
Artifacts are produced by highly oversaturated pixels in the APS. Oversaturated pixels are pixels that are exposed to more light energy than the pixel's photodetector can absorb during exposure time.
APS devices can be operated in a double-sampling mode, in which the difference between the reset voltage and the signal voltage determines the brightness of the pixel. At zero light level, the signal value does not drop from the reset voltage and hence the difference voltage is zero, producing a black pixel. As the signal voltage drops, the difference voltage increases and the pixel brightens.
At high light levels, the signal voltage saturates at a saturation voltage V.sat.
The reset voltage also drops in response to incident light but at a much lower rate than the signal voltage. At very high light levels, the reset voltage begins to decrease significantly. At extremely high light levels reset voltage R will also saturate at V.sat. At this point, the difference voltage is zero, which produces a black pixel.
This is mainly a problem in very high contrast images. The extreme brightness of the sun will cause this effect in virtually all CMOS imagers, even at very short integration times. This is a disconcerting effect in outdoor imaging situations, in which the sun is often accidentally included in the field of view.
FIG. 1 illustrates such a situation in a digital-image 10 produced using a conventional APS CMOS imager. The presence of the sun 12 in the image produces an artifact 14 in which the center of the sun appears black.